Update the project documentation with the testing guidelines

[sergei-mironov]

As a reminder I suggest updating the project docs with these:

• each piece of logic being tested should live in a separate test function
• only parametrize tests if the same logic is tested with different input values
• prefer self-contained test functions with duplicated code if necessary (within reason)
• avoid test function with too much logic, that is overly modular / parametrized, or makes heavy use of fixtures or other setup code

Originally posted by @dime10 in #98 (comment)

The closest known gudelines are https://github.com/PennyLaneAI/guidance-docs/blob/master/development/code-review.md#code-author-and-reviewers-checklist

[dime10]

Why is this a GitHub issue? Also the title is way too long

[sergei-mironov]

Just pressed the "reference in the new issue button". I believe having an issue would be a best way to both discuss and merge the guidelines. But feel free to close and organize it in another way.,

[dime10]

We could certainly use a contribution guide, which we are currently lacking 👍